docs: def find_nested(self):
          """
          Returns a dictionary mapping parent intervals to sets of
          intervals overlapped by and contained in the parent.
  
          Completes in O(n^2) time.
          :rtype: dict of [Interval, set of Interval]
          """
          result = {}
  
          def add_if_nested():
              if parent.contains_interval(child):
                  if parent not in result:
                      result[parent] = set()
                  result[parent].add(child)
  
          long_ivs = sorted(self.all_intervals, key=Interval.length, reverse=True)
          for i, parent in enumerate(long_ivs):
              for child in long_ivs[i + 1:]:
                  add_if_nested()
          return result